1. Field of the Invention
This invention relates to an improvement of a polygon mirror used in a post-objective type light deflector.
2. Description of the Related Art
Optical scanning devices used in optical systems incorporated in laser printers or the like can be divided into two main types: one is the post-objective type, in which light is focused into a beam by a focusing lens and then sent to a light deflector, and the other is the pre-objective type, in which light is first deflected by means of a light deflector and then focused by a focusing lens.
Since, in the case of the first type of scanning device, the light beam is focused before reaching the deflector, the focusing lens can therefore be small and simple in construction. However, in this case, since the beam focusing spot is generally set on a curved surface, an error in the scanning linearity and the curvature of image field will increase.
In the case of the latter type device, in contrast, the beam is first deflected by the light deflector, before being focused by the focusing lens, so that the beam spot can be easily set on a flat plane and therefore an error in the scanning linearity and the curvature of image field can be easily corrected. However, in this case, it is necessary to use a wide angle lens, such as an f.theta. lens, as the focusing lens, in order to obtain a proper deflection angle, making the lens construction complex.
As has been made clear above, each of the two types of scanning devices (post-objective type and pre-objective type) has its advantages as well as disadvantages.
In the case of the conventional post objective light deflector disclosed in Japanese Patent Disclosure (Kokai) No. 61-156020, for example, an error in the curvature of the image field is corrected by use of a polygon mirror having a spherical or cylindrical convex reflection surface with a radius of curvature, and the error in scanning linearity is corrected by a succeeding electrical correction processing (the clock frequency of the image signal is changed according to variation in the scanning speed).
However, the above conventional device has the disadvantage that it is necessary to effect electrical correction processing in order to correct the error in the scanning linearity.